In commonly owned U.S. Pat. No. 3,751,708 there has been disclosed and claimed a sweep circuit for an oscilloscope enabling simultaneous visualization of one or more signal voltages from the output of a test circuit along with a constant reference voltage with which the signal voltages are to be compared. According to that system, the beam of a cathode-ray tube forming part of an oscilloscope is electro-magnetically deflected by a triangular or ramp current in the x direction and by a sinusoidal current of substantially higher frequency in the y direction to trace an invisible raster on the screen of the tube, the beam being held suppressed until a comparator detects a coincidence between the amplitude of the y-deflection signal and the input signal to be visualized. At that point the y sweep is briefly interrupted and the beam is turned on whereby a short horizontal trace is produced on the screen. As further disclosed in that patent, a comparator receiving a constant reference signal may trigger an acceleration of the horizontal sweep, at the instant when the y-deflection signal matches the reference signal, in order to trace a base line during a cycle of the vertical sweep. Since coincidences with other input signals cannot be detected during such a cycle, the tracing of a base line in this manner is allowed to take place only on every n.sup.th cycle where n should be equal to at least 20. Because of this intermittency, the base line appears faint in comparison with the traces of the displayed test signals.
The sweep in the horizontal or x direction, in the system of U.S. Pat. No. 3,751,708, has a low repetition frequency or cadence of 1 Hz, for example, consistent with the luminous persistence of the oscilloscope screen. Naturally, the input signal to be displayed on the screen must recur with the same frequency or a multiple thereof. This prevents the simultaneous visualization of two or more periodic input signals which are not harmonically related to a common sweep frequency.